Irrespective of cost, and how they are going to pay for it (They have said they are paying for it as part of a huge government regeneration program), if it does get built, and with Lord Foster involved, it will obviously be the best stadium in the country. It might not be to everyone's taste, but as the most modern stadium built to such a size, it will become a landmark that tourists will flock to.
If I go to Madrid or Barcelona or Munich, I always fancy a trip to the stadiums because I love football and they are iconic. The same can be said about Manchester United and Old Trafford unfortunately, even in its current state.
I think I said in an earlier post, it's only a matter of time before they get a new stadium, and they aren't going to build some half hearted stadium with a similar capacity to what they have now. Biggest stadium in the country and likely to have facilities that make most of the others in the league look second rate.
If they finance it for themselves, then fair enough, and the extra income it will generate will obviously help with servicing the borrowed cost. Whether you like it or not, their brand is still huge.
Hopefully the team on the field stays shit for a long time to come.